<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            width: 200px;
            height: 40px;
            background-color: red;
            border-radius: 5px;
            /* border-top-left-radius: 20px;
            border-top-right-radius: 30px;
            border-bottom-left-radius: 40px;
            border-bottom-right-radius: 40px; */
        }

        input {
            /* border: 0; */
            /* outline: none; */
            /* border-color: yellow; */
            /* outline-color: yellow; */
            /* outline: 1px solid pink; */

        }

        /* input:focus {
            outline-color: red;
        } */
    </style>
</head>

<body>
    <form action="">
        <input id="btn" type="text" value="你好">
    </form>

    <div class="box"></div>
</body>

</html>
<script>
    document.querySelector("#btn").onfocus = function () {
        this.value = "";
    };
    document.querySelector("#btn").onblur = function () {
        this.value = "你好";
    };
</script>